The more commonly used and thus economically important sulfonated amines are those derived from aniline, the toluidines, xylidines, chloroanilines, napthylamines, aminoanthraquinones and the like. The simplest of the sulfonated aromatic amines is sulfanilic acid of the formula: ##STR1##
Millions of pounds of sulfanilic acid are consumed annually, primarily to produce dyestuffs but some is also used in the synthesis of other materials including substantial amount of phenylhydrazine-p-sulfonic acid. The major portion of the sulfanilic acid is consumed in the manufacture of fluorescent whitening agents. Such fluorescent dyestuffs are applied to paper, woven and non-woven textiles, and are components of soap and detergent laundering agents. Such fluorescent whitening agents are marketed under the Tinopal trademark. For example, in excess of a million pounds a year is used for the manufacture of one of the whiteners applied to paper. Thus, any savings effected in the production of sulfanilic acid are of major economic interest.
The most straight forward synthesis of these amines and particularly sulfanilic acid consists of reacting the amine, aniline, with sulfuric acid in substantially stoichiometric proportions to form aniline hydrogen sulfate (AHS), followed by splitting off water and the migration rearrangement of the resulting sulfonic acid group to the para position according to the following equations: ##STR2##
These reactions have been carried out directly or in the presence of inert, high boiling, water-immiscible solvents (Jabobs: Ind. Eng. Chem. 35; pg. 321-323). The syntheses using solvents produce a product of better appearance (Gardner color &lt;9) but solvent cost, handling, removal and recycling impose excessive costs that are not competitively bearable. Thus most sulfanilic acid is now prepared in the absence of solvents.
The reaction (I) between aniline and sulfuric acid to form the aniline hydrogen sulfate proceeds as rapidly as the reactants are mixed and is exothermic. The heat of the exothermic reaction causes the temperature of the reaction mass to rise. Aniline hydrogen sulfate melts at about 160.degree. C. It is very soluble in water at elevated temperatures. The temperature at which it liquefies varies with the amount of water in the mixture.
To cause water to be split off and to promote the para-rearrangement of reaction (II) at a slow but measurable rate, the minimum temperature must be about 165.degree. C. At this temperature the aniline hydrogen sulfate is slowly converted, by an endothermic reaction, to sulfanilic acid. Sulfanilic acid, (in contrast to the aniline hydrogen sulfate salt), is stable and does not melt up to its decomposition temperature which is about 280.degree. C.
It is instructive to visually observe the conversion to sulfanilic acid of a molten mass of aniline hydrogen sulfate. As stated above, when water is present in the aniline hydrogen sulfate, the mixture forms a liquid or pasty mass at temperatures below 160.degree. C. As the mass is heated, water is driven off so that substantially none is present at about 160.degree. C. When the temperature is raised to the rearrangement temperature, the conversion starts. Sulfanilic acid is insoluble in molten aniline hydrogen sulfate. During the early stages of the conversion the molten mass becomes a pasty, sticky mass of a liquid phase and a dispersed solid phase. Bubbles form in the liquid from the water vapor that evolves. As the conversion continues, the mass becomes increasingly solid with a liquid phase dispersed within it. The upper surface is no longer smooth. It assumes a pockmarked appearance from the simultaneous escape of water vapor and the solidification of that part of the mixture from which it has emerged.
By maintaining the sample above the conversion temperature for a sufficient period of time, the sample is substantially completely converted to a block of solid sulfanilic acid which strongly adheres to the surface with which it was in contact during the conversion. It is this behavior pattern of the mixture during its conversion that has led to previously employed processes briefly described below.
During this period of rearrangement and water elimination, color bodies of indeterminate composition form; causing a grayish or deep purplish product. For many uses this product, if not too intensely colored, can be used without further upgrading. It is known that iron tends to catalyze a reaction which also forms color bodies in the product.
The classical process for making sulfanilic acid is the "baking" process wherein aniline acid and sulfuric acid are mixed together to form the solid aniline hydrogen sulfate salt (Fiat Report 1313) (1945). In one variant, a batch-type process, a layer of the solid salt several centimeters thick is spread on lead or iron trays. The loaded trays are then slowly passed through an oven or tunnel kiln and heated to above the rearrangement and water elimination temperature. The eliminated water is vented from the kiln and condensed together with any vaporized aniline. When the trays emerge, conversion to sulfanilic acid is substantially complete, the trays are emptied and the cycle is repeated by reloading the trays with additional aniline hydrogen sulfate.
The molten aniline hydrogen sulfate, prior to rearrangement and water elimination is corrosive, especially to iron. In the trays, while corrosion takes place, the rate of corrosion and erosion of metal is not excessive because an adherent retarding surface coating of sulfanilic acid forms on the surface of the trays. This coating also retards migration of the colored corrosion products through the rearranging mass.
A variant of the tray "baking" process employes an elongated, rotating cylindrical steel reactor with an opening and cover plate in its center. The interior is tapered slightly so that non-sticky solids will flow towards the center opening. Flanges are attached to both ends. By means of the flanges, articulated end pieces with center holes are connected to the reactor. Hollow shafts, having openings coinciding with the center holes are attached to the end pieces. By appropriate valves the hollow shafts are used to charge liquid aniline and sulfuric acid into the reactor at one end and to vent the water vapor at the other end. By suitable gearing, the shafts and reactor are rotated. Heavy rods or steel balls or both, are placed within the reactor so that the rotating reactor behaves as product-comminuting ball or rod mill. The reactor is mounted within a furnace which supplies the heat necessary for the rearrangement reaction II and to drive off the water vapor.
Aniline and sulfuric acid, in substantailly equimolar amounts are charged to the rotating reactor to form aniline hydrogen sulfate therein. When the charging is completed, the temperature within the furnace is raised to heat the reactor and its contents above the rearrangement temperature. The released water is vented through one of the hollow shafts exiting the end of reactor. The water is externally condensed.
It is possible to follow the progress of the rearrangement and to observe its completion by following the rate and amount of water being condensed. Another, more empiric indication of the progress of the reactions is the sound emanating from the equipment. At the beginning of the reaction between the aniline and the sulfuric acid, the thudding sound of the rods dropping during the rotation can be heard. As the aniline hydrogen sulfate mass is heated and the conversion starts to take place the mixture becomes a pasty, sticky mass. The impact of the falling rods is absorbed by the pasty mass, which causes a decided dulling of the noise. As the reaction mixture appreciably converts to sulfanilic acid and becomes an increasingly dry solid mass, the character of the noise slowly reverts to the sharper impact sounds. The ultimate test indicating that the conversion to sulfanilic acid is substantially complete is the cessation of water condensation. As soon as water stops coming over, heating is stopped and the reactor is allowed to cool. The rotation of the reactor is continued for a sufficient period to break away the sulfanilic acid scale from the reactor's interior surface and to break up any large lumps. The rotation is then stopped, the cover plate in the center is removed and the product discharged from the center opening.
The rod mill reactor and the tunnel kiln process are both basically batch processes. The reactor has to be heated and then cooled. Heat transfer is poor during the period when the reaction mixture is pasty and scale is adhering to the walls. The rod mill reactor must also serve as a mill to break up the lumps and break off the scale, thereby prolonging the production cycle.
Other non-solvent processes include forming a pool of molten aniline hydrogen sulfate in which a segment of a continuously rotating drum dryer is immersed. The interior of the dryer is heated by high pressure steam or some other heat transfer fluid. The aniline hydrogen sulfate adhering to the exterior surface of the drum, after it emerges from the pool, is heated above the rearrangement temperature. The speed of rotation of the drum and the temperature to which the adherent film is raised are correlated so that rearrangement to sulfanilic acid is complete before reentry of the coated portion into the molten pool. A doctor blade is positioned so that the completely rearranged sulfanilic acid is scraped off and collected before the drum reenters the molten pool. While feasible, the parameters of temperature, rotation rate, product adherence, etc., render production control difficult so that this process is not economically viable.
Another method is based on mixing sulfuric acid, aniline and a small amount of water at about 125.degree.-145.degree. C. and spraying the resulting solution into a spray dryer along with air heated to about 400.degree. C. The temperature of the outgoing air is held at about 260.degree.-270.degree. C. The resulting air stream in which solid sulfanilic acid particles are entrained is passed through a cyclone and bag filters. This separates the air from the solids. This method requires completion of the rearrangement in a very short period--in the order of a second. Current environmental requirements are very stringent and to meet even minimal solid separation requirements of the solid product from the gases necessitates inordinately expensive arrangements and investments.
Another variant involves utliization of fluidized bed technology wherein either solid or molten aniline hydrogen sulfate is continuously fed to a fluidized bed of sulfanilic acid. The bed is fluidized by a sufficiently hot inert gas to maintain the bed above the rearrangement temperature. The water vapor, which evolves, leaves with the vented inert fluidizing gas. Sulfanilic acid is continuously or periodically withdrawn from the reaction bed. The inert gas, which may be heated air at about 200.degree. C.-250.degree. C. is not only used as the fluidizing medium but also as the heating element and the carrier for sweeping out the water. Of course, this method requires very efficient cyclones and/or bag filters and often porous metal filters fitted with cyclic filter cleansing means to remove the product from the vented gases. These are expensive so that this method is non-competitive with the batch rod mill method.